A conventional switched-mode power supply uses as a transformer an electromagnetic transformer which operates based on electromagnetic induction as its operation principle. The electromagnetic transformer has a problem under high frequencies that hysteresis loss, eddy-current loss and loss caused by skin effect are increased. Moreover, reduction in size and thickness of the electromagnetic transformer itself leads to copper loss caused by winding an ultra-fine wire a number of times, reduces magnetic coupling, and increases leakage flux. All of those may result in large decrease in the efficiency of a power supply circuit. Furthermore, the electromagnetic transformer has a problem of generation of electromagnetic noises from windings.
Therefore, piezoelectric transformers have been developed in recent years. As such a piezoelectric transformer, one is known in which an input part, an output part, and another input part are arranged in that order in the lengthwise direction of a piezoelectric substrate in which both main surfaces are rectangular; a pair of internal electrode layers for the input part are provided within the piezoelectric substrate in the input part with a predetermined interval in the thickness direction; and three or more internal electrode layers for the output part are provided within the piezoelectric substrate in the output part with a predetermined interval in the thickness direction (see Patent Documents 1 and 2).    Patent Document 1: JP-A-2001-185775    Patent Document 2: JP-A-2002-289937